Pour milk into an oven-safe 9x13-inch casserole dish. Bake 4-5 hours until a golden-brown crust forms on top of the milk.
Remove from oven and cool slightly so the crust can be removed. The milk should be a light tan. Continue to cool to 70°-77°F. (At this point, the baked milk is very sweet with caramel notes. You can stop here, if you like, and chill for a sweet, rich milk, or continue with the fermentation.)
While the milk is cooling, prepare a vessel for fermentation. A Yogotherm, or a similar type of thermos incubator, works well. Pour the baked milk into the vessel and stir in buttermilk.
Cover the container and leave it in a warm place for 10-12 hours or overnight.
Pour the finished ryazhenka into a bottle or pitcher and store in the refrigerator. It will be somewhat thick, much like drinkable yogurt.